Demagnifying projection exposure using ultraviolet light is used in lithography for manufacturing extremely fine semiconductor devices such as semiconductor memories and logic circuits.
The smallest dimension of a pattern that can be transferred by demagnifying projection exposure is proportional to the wavelength of light used in transfer and inversely proportional to the numerical aperture (NA) of the projection optical system. This means that it is necessary to shorten the wavelength of the light used to project extremely fine circuit patterns. For this reason, the wavelength of ultraviolet light used in pattern transfer has become increasingly shorter, e.g., 365 nm in mercury-vapor lamps, 248 nm in KrF excimer
As semiconductor devices are becoming smaller and smaller at an increasing rate, there is a limit on what lithography using ultraviolet light can accomplish. Accordingly, in order to burn in an extremely fine circuit pattern of less than 0.1 μm in an efficient manner, a demagnifying projection exposure apparatus using extreme ultraviolet light (EUV light) of a shorter wavelength, on the order of 10 to 15 nm, has been developed.
Since absorption by the material used is pronounced in the region of EUV light, a lens-based optical system that utilizes the refraction of light, as in the case of visible or ultraviolet light, is impractical. An exposure apparatus that employs EUV light, therefore, uses a reflection optical system. In such a case, use is made of a reflecting-type reticle in which the pattern to be transferred is formed on a mirror using an absorbing body.
A multilayer mirror and an oblique-incidence total-reflection mirror are examples of reflecting-type optical elements for constructing an exposure apparatus that relies upon EUV light. In the EUV region, the real part of the index of refraction is slightly smaller than unity and, as a result, total reflection occurs if use is made of oblique incidence in which the EUV light just barely impinges upon the mirror surface. Usually, a high reflectivity of 20 or 30% or more is obtained by oblique incidence of within several degrees measured from the surface. However, because such oblique incidence diminishes the degree of freedom in terms of optical design, it is difficult to use an oblique-incidence total-reflection mirror in a projection optical system.
A multilayer mirror obtained by building up alternating layers of two types of substances having different optical constants is used as a mirror for EUV light employed at an angle of incidence close to that of direct incidence. For example, molybdenum and silicon are formed in alternating layers on the surface of a glass substrate polished to have a highly precise surface shape. The layer thicknesses of the molybdenum and silicon are, e.g., 0.2 nm and 0.5 nm, respectively, and the number of layers is 20 each. The thickness of two layers of the different substances is referred to as the “film cycle”. In the above example, the film cycle is 0.2 nm+0.5 nm=0.7 nm.
When EUV light impinges upon such a multilayer mirror, EUV light of a specific wavelength is reflected. Only EUV light of a narrow bandwidth centered on a wavelength λ that satisfies the relationship of Bragg's equation2×d×sinθ=λwhere λ represents the wavelength of the EUV light and d the film cycle, will be reflected efficiently. The bandwidth in this case is 0.6 to 1 nm.
The reflectivity of the reflected EUV light is 0.7 at most, and the unreflected EUV light is absorbed in the multilayer films or in the substrate. Most of this energy is given off as heat.
Since a multilayer mirror exhibits more loss of light than a mirror for visible light, it is necessary to hold the number of mirrors to the minimum. In order to realize a broad exposure area using a small number of mirrors, use is made of a method (scanning exposure) in which a large area is transferred by causing a reticle and a wafer to perform scanning using fine arcuate areas (ring fields) spaced apart from the optical axis at fixed distances.
FIG. 5 is a schematic view illustrating a demagnifying projection exposure apparatus that employs EUV light according to an example of the prior art. This apparatus includes an EUV light source, an illuminating optical system, a reflecting-type reticle, a projection optical system, a reticle stage, a wafer stage, an alignment optical system and a vacuum system.
By way of example, a laser plasma light source is used as the EUV light source. In the example shown in FIG. 5, a target material (supplied from a target supply unit 502) placed in a vacuum vessel 501 is irradiated with pulsed laser light (laser light that is produced by an excitation pulse laser 503 and supplied via a condensing lens 504), a high-temperature plasma is produced and EUV light having a wavelength of, say, 13 nm that emanates from the plasma is utilized as the EUV light source. A thin film of metal, an inert gas or a droplet is used as the target material, which is fed into the vacuum vessel 501 by means such as a gas jet (the target supply unit 502). In order to increase the average intensity of the EUV light emitted, the pulsed laser should have a high repetition frequency and the apparatus should be operated at a repetition frequency of several kilohertz.
The illuminating optical system that introduces the light from the EUV light source to a reticle 550 comprises a plurality of multilayer mirrors or a plurality of oblique-incidence mirrors and an optical integrator, etc. A condensing mirror (first mirror 506 of the illuminating system) constituting the first stage functions to collect EUV light emitted from the laser plasma substantially isotropically. An optical integrator 507 functions to illuminate the reticle uniformly using a prescribed numerical aperture. An aperture (field-angle limiting aperture 510) for limiting to a circular arc the area of the reticle surface that is illuminated is provided at a conjugate point with respect to the reticle disposed in the illuminating optical system.
Reflected light from the optical integrator 507 is reflected by a second mirror 508 of the illuminating system, passes through the field-angle limiting aperture 510 and is reflected again by a third mirror 509 of the optical system, thereby arriving at the reticle 550.
The projection optical system uses a plurality of mirrors (first through fourth mirrors 511 to 514). Though using a small number of mirrors allows EUV light to be utilized very efficiently, this makes it difficult to correct for aberration. The number of mirrors needed to correct for aberration is four to six. The shape of each of the reflecting surfaces of the mirrors is convex or concave spherical, or convex or concave aspherical. The numerical aperture NA is 0.1 to 0.3.
To fabricate the mirror, use is made of a substrate consisting of a material, such as glass, having a low coefficient of expansion or silicon carbide, that exhibits a high rigidity and hardness and a small coefficient of expansion, the substrate is formed to have a reflecting surface of a predetermined shape by grinding and polishing, and multilayer films such as molybdenum and silicon are formed on the reflecting surface. In a case wherein the angle of incidence is not constant owing to the location of the layer in the mirror surface, the wavelength of the EUV light, the reflectivity of which rises depending upon the location, shifts if use is made of multilayer films having a fixed film cycle, as is evident from Bragg's equation cited above. Accordingly, it is required that a film-cycle distribution be provided in such a manner that EUV light of the same wavelength will be reflected efficiently within the mirror surface.
A reticle stage 552 and a wafer stage 562 have a mechanism in which scanning is performed synchronously at a speed ratio proportional to the reducing magnification. Let X represent the scanning direction in the plane of the reticle 550 or wafer 560, Y the direction perpendicular to the X direction, and Z the direction perpendicular to the plane of the reticle 550 or wafer 560.
The reticle 550 is held by a reticle chuck 551 on the reticle stage 552. The reticle stage 552 has a mechanism for high-speed movement in the X direction. Further, the reticle stage 552 has a fine-movement mechanism for fine movement in the X, Y and Z directions and for fine rotation about these axes, thus making it possible to position the reticle 550. The position and attitude of the reticle stage 552 are measured by laser interferometers (not shown) and are controlled based upon the results of measurement.
The positional relationship between the position of the reticle 550 and the optical axis of the projection optical system and the positional relationship between the position of the wafer 560 and the optical axis of the projection optical system are measured by an alignment detection mechanism that includes alignment detecting optical systems 553, 563, and the positions and angles to the reticle stage 552 and wafer stage 562 are set in such a manner that the projected image of the reticle will coincide with a predetermined position on the wafer.
Further, the focus position along the Z axis on the wafer surface is measured by a focus-position detecting mechanism that includes a focus detecting optical system 564, and the position and angle of the wafer stage 562 are controlled. During exposure, therefore, the surface of the wafer is always maintained at the position at which the image is formed by the projection optical system.
When one scanning exposure of the wafer ends, the wafer stage 562 is stepped in the X and/or Y directions to move the stage to the starting position of the next scanning exposure, then the reticle stage 552 and wafer stage 562 are again scanned synchronously in the X direction at a speed ratio that is proportional to the magnification of the projection optical system.
Thus, an operation for synchronously scanning the reticle and wafer in a state in which the demagnified projection image of the reticle is formed on the wafer is repeated (by a step-and-scan operation). The transfer pattern of the reticle is thus transferred to the entire surface of the wafer.
In the conventional EUV exposure apparatus, the following problems arise in a case where consideration is given to TTL (Through-the-Lens) alignment in which the relative positioning (referred to as alignment below) between a reticle and a wafer is carried out via a reflecting-type reticle and multilayer mirrors using non-exposing light:
Since the reflecting-type reticle and multilayer mirrors are optimized to obtain a high reflectivity with EUV light, sufficient reflectivity is not obtained with regard to alignment light that is non-exposing light, and it is conceivable that highly precise alignment will not be achieved. Accordingly, there is a need of an optical system in which satisfactory alignment signal light is obtained at all times in order to achieve alignment.
Furthermore, the alignment detecting optical system must take into consideration space-relation limitations. For example, the exposing light must not be blocked when alignment detection is not carried out. As a consequence, a limitation is imposed upon the structures of the detection system and detection optical system.
In the ordinary projection exposure apparatus, a method of aligning the reticle and wafer via a projection lens is referred to as TTL alignment. In an EUV exposure apparatus, however, the projection optical system is constituted not by lenses but by the multilayer-mirror optical system. It is difficult to refer to this scheme as a TTL scheme. However, for the sake of simplifying the description, an alignment system that uses the intervention of a multilayer-mirror optical system will also be defined as being a TTL alignment scheme in this specification.